After a well bore has been drilled and lined with casing it is the usual practice to cement the casing in place to prevent migration or channeling of water or other fluids along the outer side thereof. Conventional cementing techniques involve displacing cement slurry down through the bore of the casing and out a shoe on the bottom thereof so that the cement fills the annulus between the casing and the well bore wall. A sufficient volume of slurry is displaced so that the top of the cement in the annulus extends several hundred feet up inside the intermediate string of casing.
A bottom plug with a rupture disc or the like is run ahead of the cement column in the casing and a displacement plug is run at the upper end of the column to separate the cement and the displacement fluids. When the bottom plug reaches the shoe at the bottom end of the casing, pressure is used to rupture the disc so that the slurry can be pumped out into the lower end of the annulus. When the top or displacement plug reaches the shoe, most all of the slurry will have been pumped into the annulus. Once the cement has set up or hardened, perforations are shot at one or more intervals in the casing in order to communicate oil-bearing formations with the bore of the casing so that the well can be placed on production.
Although the foregoing cementing technique has been used for many years, it has a number of shortcomings. The process is time consuming because the cement must be pumped all the way to the bottom of the casing and then back up into the annulus. Expensive chemicals must be used to retard set up of the cement. A large amount of very expensive equipment also is necessary. These factors make cementing a very high cost process which adds considerably to the total completion costs of a well.
An object of the present invention is to provide a new and improved well cementing system that obviates the foregoing shortcomings of prior techniques.
Another object of the present invention is to provide a new and improved well cementing system where the slurry is displaced down the annulus instead of down through the casing, which saves considerable time and expense to complete a cementing job.
Still another object of the present invention is to provide a new and improved cementing system of the type described which employs unique cementing shoe and check valve structure that are particularly adapted for use in the a reverse cementing process of the present invention.